1. Field of the Invention
The invention relates to a control device for a low-pressure fluorescent lamp.
2. Discussion of the Related Art
The electrical behavior of these fluorescent lamps which contain low-pressure gases (neon, argon) is similar to that of a zener (avalanche) diode with a resistance in the gas that may become very low and negative after breakdown. Ions moving at high speeds lead the atoms of the gas to assume excited states in which they give out luminous lines.
The system used to control the lamps typically comprises a control device with a current source and an oscillating circuit in which the lamp is placed. This oscillating circuit typically has an inductor and a very large capacitor series-connected with the lamp and a very small capacitor parallel-connected with the lamp. This system enables current discharges to be made to go into the lamp between these two electrodes, in one direction and then in another, thus preventing the migration of ions. The current in each of the directions corresponds to one alternation of the oscillating circuit, such that each alternation corresponds to one half cycle of the oscillation of the current supplied to the oscillating circuit and is therefore approximately equal to one half of the period of oscillation.
According to the prior art, the control device with a current source typically has two electronic switches using power transistors supplied with a high DC voltage and a current transformer. The current transformer is preferably a saturation transformer that limits the current in the lamp by the saturation of its magnetic core and leads to the switch-over of the switches.
The electronic switches generally use bipolar technology power transistors for the switching and parallel and reverse-connected diodes to let through the current during the alternations and various protection elements such as diodes and capacitors.
These transformer devices are very bulky and costly because they require many components and allow only a very low degree of integration. Furthermore, the storage time of the bipolar transistors is a highly variable characteristic, for example ranging from 2 to 7 microseconds. This variation is not negligible as compared with the time at the end of which the transformer gets saturated for a current alternation. It is about three microseconds for an alternation time of about ten microseconds. Hence, the time at the end of which the bipolar transistor goes off after saturation of the transformer in an alternation varies from 5 to 10 microseconds. This is very troublesome. In practice, the storage time of each transistor is measured after manufacture to classify it in a group corresponding to a narrow range of values, in order to use it in a control device adapted by means of resistors to this range of values. All this entails heavy penalties and is very costly.
To light up the lamp, the characteristics of the oscillator circuit with a parallel-connected inductor and capacitor are used. When this oscillating circuit works at its resonance frequency, its characteristic impedance becomes very small. The current in the oscillating circuit therefore becomes very great and the voltage in the parallel-connected capacitor also becomes very great. This is the principle used to break down the gas in a low-pressure fluorescent lamp. It has been seen that the oscillating circuit has a very small parallel-connected capacitor and a very large series-connected capacitor. When the lamp is not lit, at the start of the operation for turning it on, it is equivalent to an open circuit. If the inductor is referenced L, the series-connected capacitor is referenced Cs and the parallel-connected capacitor is referenced capacitor Cp. The resonance frequency f0 at the starting up of the system formed by the oscillating circuit and the lamp is given by f0=1/2.pi.(L.Cp.Cs/(Cp+Cs)).sup.1/2. Since Cp&lt;&lt;Cs, we have f0 approximately equal to 1/2.pi.(L.Cp).sup.1/2.
If the operation is done at this resonance frequency f0, it has been seen that the current becomes very high in the oscillating circuit and an overvoltage then appears at the parallel capacitor Cp and therefore between the two electrodes of the lamp: at each alternation, the voltage rises to reach a voltage that is high enough (about 1,200 volts) to cause the breakdown of the gas. The lamp, as we have seen, is then equivalent to a very low value resistor that lets through all the current: the parallel-connected capacitor Cp is then virtually short-circuited and the new resonance frequency f1 of the system formed by the oscillating circuit and the lamp is then given by f1=/2.pi.(L.Cs).sup. 1/2, f1 being far lower than the first resonance frequency f0 (for Cp&lt;&lt;Cs).
However, much as it is worthwhile making the oscillating circuit work at its resonance frequency f0 when starting up the system in order to achieve gas breakdown and therefore to light up the lamp, it is also equally dangerous to then continue to work at the resonance frequency f1 which is far lower.
Indeed, for this new resonant system, without the parallel-connected capacitor, the characteristic impedance is also far lower because of the large series-connected capacitor Cs. The current then becomes far greater with the risk of disrupting the system.